1. The Field of the Invention
Dental bleaching and dental bleaches are disclosed. Discussion is provided concerning viscous dental bleaches which may be placed into a tray or otherwise held against a patient's teeth in order to cause a lightening, whitening and stain removal effect without damaging the enamel of the patient's teeth.
2. The Relevant Technology
The ability to whiten both vital and non-vital teeth with peroxides has been known for well over 40 years. By nature, the human race is very diversified genetically as to tooth shape and color. The color of teeth within the human race usually ranges from whites, blacks, greys, browns and yellows. Environment, nutrition, medication and diet can affect tooth color. Some foods such as blueberries, cranberries, coffee and tea can significantly alter a person's tooth color from its original state. Other habits like smoking or chewing of tobacco can darken a person's teeth over a period of time. Tetracycline and other medications can have a darkening or graying effect on teeth. Children born to women who ingested tetracycline during pregnancy often have permanently stained teeth. Tetracycline staining may come in definite unsightly bands on the teeth.
A desire among the populace for bright white teeth has increased as economic stature and standards of living have improved. In developing and developed nations, social standing, personal relations and career opportunities can be positively or negatively influenced by the appearance of one's teeth. Generally, it is preferred to have white teeth rather than to have dark teeth. Since whiter teeth are generally more desirable, many materials have been devised to whiten an individual's teeth by chemical means.
The most commonly accepted chemicals used in teeth whitening today are peroxides. Peroxides are known in the art as oxidizing agents and are highly reactive. Organic molecules that reflect specific wavelengths of light back to our eyes are usually very complex molecules. These colorful substances are usually called dyes and pigments. Oxidation of these organic dyes and pigments usually destroys their ability to absorb light and therefore renders them white. For example, the paper industry uses hydrogen peroxide to bleach brown wood pulp to white wood pulp.
Peroxides are a safe teeth whitener, because they are generally deemed safe from a physiological standpoint as compared to other oxidizing agents. The peroxides of choice for teeth whitening are: hydrogen peroxide, carbamide peroxide, sodium perborate, and sodium percarbonate. When these peroxides are in appropriate contact with teeth they will usually oxidize both internal and external organic stains, rendering the teeth whiter. In contrast, inorganic stains are usually not affected by peroxides. Individuals with predisposed inorganic stains usually will not experience a whitening effect with the application of peroxides. However, the majority of the world's human population will experience a whitening effect through the application of peroxides to teeth.
Since the use of peroxides has been generally accepted for use in teeth whitening, a multitude of methods for applying peroxides have been devised. One method of application is to paint a peroxide in water solution directly on the teeth. A problem with water solutions of peroxides is that they are too thin or runny. This causes them to run off of the teeth due to the force of gravity and run onto the soft tissues of the mouth. High concentrations of peroxides are painfully irritating to soft tissue, causing serious discomfort to a patient. The method of painting a water solution of peroxide on a person's teeth is usually an “in-office” procedure, because of the inherent difficulties associated with patients applying it to their own teeth. In most cases, the patient's lips are painfully retracted during the entire treatment, and the patient is confined to sitting in the dental chair. The danger of the peroxide solution running off the teeth and contacting soft tissue is always present when peroxide and water bleaching solutions are used.
Patient comfort during the bleaching treatment may be improved by shortening the time of bleaching. In general there are three ways to bleach teeth faster. The first is to increase the concentration of the peroxide. The second is to increase the pH of the peroxide with a basic substance. Basic substances such as sodium hydroxide will destabilize peroxide solutions, therefore making the peroxide more reactive so that it carries out its whitening effect more quickly. The third way to bleach teeth more quickly is to speed up the reaction process by heating the peroxide solution. Heat accelerates almost all chemical reactions, including bleaching. In order to reduce bleaching treatment time, any one or combination of the above methods can be implemented or augmented.
There are physiological problems associated with speeding up the bleaching process, however. First, as peroxide concentration in the bleach is increased, the bleach is more irritating to soft tissues. Second, as the pH level is increased in the bleach, the bleach becomes more caustic to soft tissue. Third, as temperature of the bleach is increased, the bleaching process is more likely to burn soft tissue or even kill teeth.
The runniness of aqueous peroxide solutions and the problems associated with accelerating the bleaching process incentivized the dental care provider to isolate soft tissues from the dental bleach. This can be accomplished by methods known in the dental profession such as use of a rubber dam. First, the patient's lips are painfully retracted, followed by placing a pre-punched rubber sheet or dam over each individual tooth. Placing a rubber dam on each tooth is slow and does not guarantee a perfect seal against leakage of the peroxide solution onto soft tissue. In order to satisfy patient demand for rapid and complete bleaching of teeth, the dental practitioner must increase risk to the patient by use of more concentrated bleaches. In order to treat teeth with a bleach that would essentially be non-irritating to soft tissues would take 2-5 full days of bleaching to see significant improvement in whitening. A patient would be very uncomfortable sitting in a dental chair with their cheeks retracted for that period of time, and the cost of such treatment would be prohibitive.
Recognition of these inherent problems associated with the “paint-on” method of bleaching with aqueous peroxides brought about significant improvements in the art of tooth bleaching. The improvements came by adding viscosity-building chemicals to the peroxide solutions. By increasing the viscosity of bleaching solutions, the ability of the bleach to flow, run or drip decreased. Substances such as glycerin, high molecular weight polyethylene glycol, fumed silica, high molecular weight polypropylene glycol, xanthan gum, hydroxy propyl cellulose and carbomer (marketed under the trade name CARBOPOL®), have generally been used to increase the viscosity of peroxide solutions.
In order to also reduce the detrimental effects of bleaching gels coming into contact with soft tissues, the pH of the peroxide solution was adjusted to around pH=7. When the concentration of peroxide was reduced, patients were required to keep the bleaching gel in contact with their teeth for a longer period of time in order to achieve the desired whitening result. This consideration was addressed by use of a dental tray which holds the bleach in contact with teeth, but which prevents the bleach from flowing away from the teeth to contact soft tissues. Use of a dental tray permits the bleach to remain in intimate contact with teeth for long periods of time without requiring the patient to sit in a dental chair with retracted cheeks. When a dental tray is used to accommodate long periods of exposure of bleach to teeth, lower concentrations of peroxides in bleach may also be used, therefore reducing the risk to soft tissue. A dental tray is an arch-shaped container which holds the bleaching material against the teeth. The dental tray also acts as a barrier against dilution of the bleach by saliva and the eventual swallowing of the bleaching material in a short period of time.
The viscosity-building material used in almost all bleaching gels today is carbomer (CARBOPOL®), manufactured by B.F. Goodrich. CARBOPOL® is a modified polyacrylic acid hydrophilic polymer, capable of forming viscous gels at concentrations above as little as 5% by weight. CARBOPOL® is the material of choice for current bleach manufacturers because it thickens peroxide solutions to a point where they will not run out of a dental tray or away from the teeth to soft tissue areas. This allows the bleach to stay in contact with the teeth for extended periods of time and protects soft tissues. The use of a dental tray and a viscous bleach allows a low concentration bleach to effectively whiten a person's teeth over a 1-2 week period of time with minimal risk to the patient. CARBOPOL® is generally the only material of choice that delivers the required high viscosity properties for a 4-8 hour bleaching period.
Fumed silica is an alternative thickening agent, but it is considered a poor choice for use in dentistry. Fumed silica is not soluble in peroxide solutions; therefore, it only suspends in the solution. At higher loadings of silica, peroxide solutions turn into a putty instead of a viscous gel. A putty suffers from not being able to flow freely around the teeth to adequately bleach them. Silica also absorbs peroxide solutions, thus binding the peroxide so that it is not as available for bleaching. Silica can also act disadvantageously as a wick to peroxide solutions. Silica-thickened bleaches dry up very quickly when placed on the teeth, and it is well known that dry peroxides do not bleach as effectively as aqueous peroxides. It is therefore generally concluded that silica is a poor choice as a thickener for tray bleaching.
There are other natural gums that could also be considered as a thickener for tray bleaching. Materials such as xanthan gum, pectin, guar gum and hydroxy propyl cellulose have been considered in the past. Natural gums are also poor materials of choice for use as thickeners in dental bleaching, although they are used widely in the food industry as thickeners. Natural gums at low concentrations are adequate for thickening bleaches that are still runny and able to drip when poured. Natural gums at high concentrations tend to turn into gelatinous masses. Gelatin does not flow and tends to clump together, thus limiting its ability to adequately flow around the teeth to effectively bleach. For this reason it is not used as the sole thickener in viscous bleaches. It has generally been concluded by the industry that natural gums are not desirable for use in tray bleaching.
Other thickening agents used in the dental industry are high molecular weight water soluble waxes, such as polyethylene glycol and polypropylene glycol. Water soluble waxes are not used as the sole thickener for peroxide solutions, however, because they do not thicken adequately. At high loadings of water soluble waxes, bleaches are still runny and maintain no gel-type properties. Bleaches made from water soluble waxes are not highly viscous and can easily escape out of the tray. It is generally known that water soluble waxes used to thicken peroxide solutions are not desirable for tray bleaching.
Considering the shortcomings of the various thickeners, CARBOPOL® remains the best compromise as a thickener for tray bleaching. CARBOPOL® has more or less the desired thickening properties to deliver a viscous bleaching gel. For this reason it is widely used as the thickener of choice in almost all currently marketed available tray bleaching materials.
CARBOPOL®, though popular, has shortcomings as well. CARBOPOL® is a cross-linked polyacrylic acid. A polyacrylic acid has the structure —CH2CH(CO2H)—. When CARBOPOL® is dispersed in water, the resulting mixture becomes acidic. Acidic substances have the ability to remove cations from inorganic matrixes to form a salt. The enamel that covers the outer portion of human teeth is composed of calcium hydroxyl apatite. Calcium hydroxyl apatite is a crystalline material similar to bone ceramic. Acidic substances like vinegar or lemons can remove enamel by forming a salt with the calcium in our enamel. It is well known in the dental industry that individuals who suck on lemons can literally etch large portions of enamel off their teeth.
Similarly, CARBOPOL® is a long chain of repeating acids, and when dispersed in an aqueous solution, it can acidically remove calcium from teeth and therefore remove tooth enamel. Since CARBOPOL® bleaches are intended for long-term bleaching regimes, keeping the bleach in contact with the tooth for more than just a few minutes, they may remove enamel during these extended bleaching sessions. It is accepted in the dental industry that the removal of layers of enamel is harmful to an individual's dental health and can lead to mottling (wearing away) of the teeth.
Manufacturers of dental bleach containing CARBOPOL® use bases to raise the pH of the bleaching material. The bases of choice generally have been sodium hydroxide, potassium hydroxide and triethanolamine. Bases are used to raise the pH of the dental bleach to less acidic levels to reduce removal of enamel by acidic etching. Since CARBOPOL® is known to be more stable in acidic ranges, almost all manufacturers of tray bleaching systems adjust their CARBOPOL® bleaches to a pH range of 5.5-6.5. The lower the pH of the bleach, the more enamel is removed by acidic etching. Long-term bleaching only lengthens out the acidic etching process. Even a bleach pH of 6.0 can remove tooth enamel during bleaching.
Additionally, it is difficult to achieve consistent pH from batch to batch of dental bleach. Because of the variations in the average molecular weight of CARBOPOL®, specially tailored quantities of base must be added to separate batches to attain a constant pH level in the dental bleach product. But it is a common practice for manufacturers to add a standard amount of base to each batch of bleach, resulting in pH variability from batch to batch. This variability will always exist even if the manufacturer intends his bleach to be at pH 7.
Another problem in using CARBOPOL® is that it has long chains of polyacrylic acid that sometimes fold around themselves to form a ball. When this happens, the inner protected parts of the CARBOPOL® chain wet at a much slower rate than the outer parts of the chain. This means that the pH of the CARBOPOL® and hence the dental bleach will vary with the total wetting time. Consequently, CARBOPOL® bleaches may have a different pH at one week after manufacture than on the day of manufacture. The only way to rectify this problem is for the dental bleach manufacturer to keep the CARBOPOL® bleach in storage for a period of time to allow pH stabilization, followed by a final pH adjustment. Such a practice is not economical. All these factors make it extremely difficult to manufacture CARBOPOL® dental bleach that has a consistent pH level both from batch to batch and over time.
Acidic etching, as explained above, is not the only cause of calcium being removed from tooth enamel. Organic acids can also remove cations by the process of chelation. Carboxylic acids have an affinity to form a salt with cations. This affinity for cations varies by the type of cations. For example, a carboxylic acid can form a salt with an element in the alkaline family, such as lithium, sodium, or potassium. The affinity to form an ionoic bond with an alkaline element is moderate. This we know because most organic acid-alkaline salts will ionize (dissolve) in water. However, the affinity of a carboxylic acid to an alkaline earth element such as beryllium, magnesium or calcium is much stronger. The affinity of an alkaline earth element to an organic acid is very strong, because most organic acid-alkaline earth salts do not readily ionize in water. Organic acids therefore have a higher affinity to bind with calcium than they do to sodium or potassium. This same principle is used during crown cementation with glass ionomer cements. Crown cementation is achieved because the polyacrylic acid (whether neutralized or un-neutralized) reacts with the alkaline earth cations of the ion leaching glass. Thus, organic acids, initially neutralized with sodium hydroxide to form a sodium salt, would switch to a calcium salt if calcium hydroxide were added to the mixture.
CARBOPOL® dispersed into water cannot be neutralized by calcium hydroxide without precipitating the polymer. Therefore, there are no CARBOPOL® bleaches neutralized with calcium hydroxide. Almost all CARBOPOL® bleaches are pH adjusted with sodium or potassium hydroxide.
There is a constant battle going on between the alkaline salts of CARBOPOL® and the calcium of the enamel. The carboxylic acid-alkaline salts of the CARBOPOL® have a higher binding power for the calcium of the enamel. Therefore, through the process of chelation, calcium is constantly being removed from the tooth enamel by CARBOPOL®-based bleaches.
Although CARBOPOL® is very effective in creating viscous gels for tray bleaching of teeth, its side effects in acid etching of tooth enamel and chelation are damaging to the very teeth that it is desired to restore to a more aesthetic condition. Information concerning the detrimental effects of prior art dental bleaches on tooth enamel can be found in the following articles: (i) Perdigao, J., et al., “Ultra-Morphological Study of the Interaction of Dental Adhesives with Carbamide Peroxide-Bleached Enamel”, American Journal of Dentistry, vol. II, No. 6, pp. 291-301, December 1998; (ii) Pinheirojunior, E. C., et al., “In Vitro Action of Various Carbamide Peroxide Gel Bleaching Agents on the Microhardness of Human Enamel”, Braz. Dent. J., 7(2): 75-79 (1996); (iii) Shannon, et al., “Characterization of Enamel Exposed to 10% Carbamide Peroxide Bleaching Agents”, Quintessence International, vol. 24, no. 1, pp. 39-44 (1993); (iv) Bitter, N., “A Scanning Electron Microscope Study of the Long-Term Effect of Bleaching Agents on the Enamel Surface In Vivo”, General Dentistry, pp. 84-88, (January-February 1998). The prior art shows a need for dental bleach and a method for its use that includes a thickener or gelling agent that does not attack or react with tooth enamel.